m52-bubble vignetting

[alacant]

Hi everyone. Gibbous moon notwithstanding... I'm having to cut about 200px from one edge of the stack due the effect you can see. I don't get this on my refractor  which leads me to believe it's something to do with the collimation; 6" f5 newtonian, canon 700d. I'm pleased, as the processing with the reflector -especially the colour- is much easier. Everything seems collimated and the images are clean and without distortions. Could anyone help me  diagnose the FWOABW, vignetting? The flats deal fine with the other edge, just not the edge in question. Any comments on the image too most welcome. I know I'm gonna need a lot more snaps to stack and a new moon. TIA and clear skies.

[Thalestris24]

Hi

Flat frames usually deal with vignetting. Are you using an M48 T-ring - that might help.

Louise

[alacant]

Hi. Thanks for the reply. Yes: M48. It looks something other than vignetting, not that usual gradual dimming toward the corners. This seems smallar but more abrupt.

[alacant]

Hi. OK. Found it. It's the coma corrector. Normal vignetting without it. It's the SkyWatcher model. Any ideas? TIA.

[Thalestris24]

7 minutes ago, alacant said:

Hi. OK. Found it. It's the coma corrector. Normal vignetting without it. It's the SkyWatcher model. Any ideas? TIA.

Ah, ok. I'd have thought the sw would be ok if the 700d is APS-C, but I've no experience of it. Sorry I can't help.

Louise

[alexbb]

40 minutes ago, alacant said:

Hi. Hands on via plate solving: my 150/750 reflector becomes 150/681 with the sw cc. It causes seemingly incurable vignetting. More likely to be my collimation though. HTH.

As you quoted in the topic I asked for SW CC info, since the CC reduces the effective FL, it actually shrinks the cone of light and it concentrates it on a smaller area. It is still possible that the secondary mirror is not aligned on the central axis or it might be too small. I'm thinking myself to replace my secondary with a larger one on my 130PDS, but for now I'm focusing more on shorter FLs.

Not sure if helpful or I made it worse,

Alex

PS. You could alter the distance between the CC and the sensor in order to avoid reducing the FL. Maybe you will get rid of the vignetting too.

[Gerry Casa Christiana]

A couple of ideas worth trying. Is your Newtonian flocked? Looks like sky glow. Also the back of the Newtonian do you have any light there? Even the computer brings light in at the back of the mirror. 

Let us know when you have the solution 

Gerry

[alacant]

Hi. The secondary is 55mm. I can't get anything more collimated and centralised than it already is. I mal-adjust and dismantle, realign and still end up with the vignetting. Dunno. Maybe others are getting this effect but ignoring it? Does anyone have a flat they could post with a 150/750 and the skywatcher cc? Cheers, clear skies and TIA.

[alacant]

d

[Astrosurf]

Just had to say that's a lovely image!

Alexxx

[alacant]

I don't think it's then cc. Here's a snap with nothing attached with the camera pointed at a white wall. It must be the camera, surely. With m42 fittings, you don't see it probably because of the smaller FOV. Visible with m48 and wider. Anyone?